I must say, on this one, I agree with jimbo to some extent. its not that all muslims knew, but many surely did. there were plenty of stories on cell phone intercepts of muslims here in the US with statements like "it's on", which of course meant nothing before the fact.
to be honest, I think the full extent of this was covered up for politically correct reasons. the extent to which US muslims sympathize, if not directly support, terrorism, if known to all, I believe would shock us.
I doubt that it would " shock " anyone now, nor right after 9/11.
There was an awful lot of stories, after the fact, about people knowing...Jews, Arabs, Muslims of all ethnicities, and most, if not all, were proven false.
It's the stuff that WAS NOT written about, that is true and worrisome, such as the blind shiek's flock and they weren't the ones talking! I think that THEY knew, FWIW.